Semiconductor laser element and method for manufacturing the same

ABSTRACT

A semiconductor laser element includes an n-side semiconductor layer, an active layer, and a p-side semiconductor layer. A least a portion of the p-side semiconductor layer forms a ridge projecting upward. The p-side semiconductor layer includes an undoped first part, an electron barrier layer containing a p-type impurity and having a larger band gap energy than the first part, and a second part having at least one p-type semiconductor layer. The first part includes an undoped p-side composition graded layer in which a band gap energy increases towards the electron barrier layer, and an undoped p-side intermediate layer disposed on or above the p-side composition graded layer. A lower end of the ridge is positioned at the p-side intermediate layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent ApplicationNo. 2019-005760, filed on Jan. 17, 2019, and Japanese Patent ApplicationNo. 2019-141147, filed on Jul. 31, 2019, the disclosures of which arehereby incorporated by reference in their entireties.

BACKGROUND

The present disclosure relates to a semiconductor laser element and amethod for manufacturing the same.

Today, semiconductor laser elements having nitride semiconductors(hereinafter also referred to as “nitride semiconductor laser elements”)can emit a spectrum of light from ultraviolet to green, and are utilizedin a broad range of applications besides light sources for opticaldisks. For such semiconductor laser elements, a structure in which ann-side clad layer, an n-side optical guide layer, an active layer, ap-side optical guide layer, and a p-side clad layer are disposed on asubstrate in that order has been described. See, for example, JapanesePatent Publication No. 2003-273473, Japanese Patent Publication No.2014-131019, and PCT Publication No. 2017/017928.

SUMMARY

In some semiconductor laser elements, a p-type impurity such as Mg isdoped to the p-side semiconductor layer of a nitride semiconductor laserelement, but p-type impurities form deep-level traps, which cause lightabsorption to occur. For this reason, the higher the light intensity inthe p-type impurity-containing layer, the larger the resultingabsorption loss, which lowers efficiency such as slope efficiency.Accordingly, the present disclosure proposes a semiconductor laserelement capable of reducing absorption losses and increasing efficiency.

In one embodiment, a semiconductor laser element includes: an n-sidesemiconductor layer formed of a nitride semiconductor; an active layeron or above the n-side semiconductor layer and formed of a nitridesemiconductor; and a p-side semiconductor layer disposed on the activelayer, formed of a nitride semiconductor and including an undoped firstpart disposed in contact with an upper face of the active layer andincluding at least one semiconductor layer, an electron barrier layerdisposed in contact with an upper face of the first part, containing ap-type impurity, and having a band gap energy that is larger than a bandgap energy of the first part, and a second part disposed in contact withan upper face of the electron barrier layer and including at least onep-type semiconductor layer containing a p-type impurity. At least aportion of the p-side semiconductor layer forms a ridge projectingupward and having an upper face and a lower end. The first part includesan undoped p-side composition graded layer in which a band gap energyincreases towards the electron barrier layer, and an undoped p-sideintermediate layer disposed on or above the undoped p-side compositiongraded layer. The lower end of the ridge is positioned at the undopedp-side intermediate layer.

In another embodiment, a semiconductor laser element includes: an n-sidesemiconductor layer formed of a nitride semiconductor; an active layerdisposed on or above the n-side semiconductor layer and formed of anitride semiconductor; and a p-side semiconductor layer disposed on theactive layer, formed of a nitride semiconductor, and including anundoped first part disposed in contact with an upper face of the activelayer and including at least one semiconductor layer, an electronbarrier layer disposed in contact with an upper face of the first part,containing a p-type impurity, and having a band gap energy that islarger than a band gap energy of the first part, and a second partdisposed in contact with the upper face of the electron barrier layerand including at least one p-type semiconductor layer containing ap-type impurity. At least a portion of the p-side semiconductor layerforms a ridge projecting upward and having an upper face and a lowerend. The second part has a thickness that is smaller than a thickness ofthe first part. The lower end of the ridge is positioned at the firstpart.

In another embodiment, a method for manufacturing a semiconductor laserelement includes: forming an n-side semiconductor layer on or above asubstrate; forming an active layer on or above the n-side semiconductorlayer; forming an undoped first part on an upper face of the activelayer, the undoped first part including at least one semiconductorlayer; forming an electron barrier layer on an upper face of the firstpart, the electron barrier layer being doped with a p-type impurity andhaving a band gap energy that is larger than a band gap energy of thefirst part; forming a second part on an upper face of the electronbarrier layer, the second part including at least one p-typesemiconductor layer doped with a p-type impurity; and forming a ridgeprojecting upward by partially removing a portion of the p-sidesemiconductor layer including a portion of the first part, a portion ofthe electron barrier layer, and a portion of the second part. Theforming the undoped first part includes: forming an undoped p-sidecomposition graded layer in which the band gap energy increases awayfrom the active layer, and forming an undoped p-side intermediate layerabove the p-side composition graded layer. In the forming the ridge, thep-side semiconductor layer is partially removed such that a lower end ofthe ridge is positioned at the p-side intermediate layer.

In another embodiment, a method for manufacturing a semiconductor laserelement includes: forming an n-side semiconductor layer on or above asubstrate; forming an active layer on or above the n-side semiconductorlayer; forming an undoped first part on an upper face of the activelayer, the undoped first part including at least one semiconductorlayer; forming an electron barrier layer on an upper face of the firstpart, the electron barrier layer being doped with a p-type impurity andhaving a band gap energy that is larger than a band gap energy of thefirst part; forming a second part on an upper face of the electronbarrier layer, the second part including at least one p-typesemiconductor layer doped with a p-type impurity; and forming a ridgeprojecting upward by partially removing a portion of a p-sidesemiconductor layer including a portion of the first part, a portion ofthe electron barrier layer, and a portion of the second part. In theforming the second part, the second part is formed so as to have athickness that is smaller than a thickness of the first part. In theforming the ridge, the p-side semiconductor layer is partially removedsuch that a lower end of the ridge is positioned at the first part.

According to certain embodiments, a semiconductor laser element can beprovided that has reduced absorption losses and, therefore, increasedefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor laserelement according to one embodiment of the present invention.

FIG. 2A is a schematic diagram of an example of the layer structure ofthe p-side semiconductor layer of the semiconductor laser element shownin FIG. 1.

FIG. 2B is a schematic diagram of another example of the layer structureof the p-side semiconductor layer of the semiconductor laser elementshown in FIG. 1.

FIG. 2C is a schematic diagram of another example of the layer structureof the p-side semiconductor layer of the semiconductor laser elementshown in FIG. 1.

FIG. 2D is a schematic diagram of another example of the layer structureof the p-side semiconductor layer of the semiconductor laser elementshown in FIG. 1.

FIG. 3A is a schematic diagram of an example of the band gap energy ofthe uppermost layer of the first part, the electron barrier layer, andthe lowermost layer of the second part.

FIG. 3B is a schematic diagram of another example of the band gap energyof the uppermost layer of the first part, the electron barrier layer,and the lowermost layer of the second part.

FIG. 3C is a schematic diagram of another example of the band gap energyof the uppermost layer of the first part, the electron barrier layer,and the lowermost layer of the second part.

FIG. 4 is a schematic diagram of an example of the layer structure ofthe n-side semiconductor layer of the semiconductor laser element shownin FIG. 1.

FIG. 5 is a partially enlarged view of the p-side composition gradedlayer and the vicinity in the semiconductor laser element shown in FIG.1.

FIG. 6A is a flowchart of a method for manufacturing a semiconductorlaser element according to one embodiment of the present invention.

FIG. 6B is a flowchart of an example of process S103. FIG. 7 is a graphshowing the relationship between the thickness of the first part and theleakage of light to the second part in Calculation Examples 1 to 5.

FIG. 8 is a graph showing the I-L characteristics of the semiconductorlaser elements of Comparative Examples 1 to 4.

FIG. 9A is a graph showing the I-L characteristics of the semiconductorlaser elements of Examples 1 to 3.

FIG. 9B is a graph showing the I-V characteristics of the semiconductorlaser elements of Examples 1 to 3.

FIG. 10A is a graph showing the I-L characteristics of the semiconductorlaser elements of Examples 3 to 5.

FIG. 10B is a graph showing the I-V characteristics of the semiconductorlaser elements of Examples 3 to 5.

FIG. 11A is a graph showing the I-L characteristics of the semiconductorlaser elements of Examples 3 and 6.

FIG. 11B is a graph showing the I-V characteristics of the semiconductorlaser elements of Examples 3 and 6.

DETAILED DESCRIPTION

Certain embodiments of the present invention will be explained belowwith reference to the drawings. The embodiments described below,however, are examples giving shape to the technical ideas of the presentinvention, and the prevent invention is not limited to the embodimentsdescribed below. In the explanation below, moreover, the samedesignations and reference numerals denote the same or similar membersfor which the detailed explanation will be omitted as appropriate.

FIG. 1 is a schematic cross-sectional view of a semiconductor laserelement 100 according to one embodiment of the present invention,showing a cross section in the direction perpendicular to the resonatorof the semiconductor laser element 100. FIGS. 2A to 2D are schematicdiagrams of the layer structures of the p-side semiconductor layer 4,each showing a different example. FIGS. 2A to 2D schematically show therelative magnitudes of the band gap energy of a portion of the activelayer 3 and each layer in the p-side semiconductor layer 4 of thesemiconductor laser elements 100. In FIGS. 2A to 2D, a one-dot chainline indicates the position of the lower end of the ridge 4 a. The lowerend of the ridge 4 a refers to the region between the lowermost sides ofboth lateral faces of the ridge 4 a. FIG. 4 is a schematic diagram of anexample of the layer structure of the n-side semiconductor layer 2.

As shown in FIG. 1, the semiconductor laser element 100 has an n-sidesemiconductor layer 2, an active layer 3, and a p-side semiconductorlayer 4, each made of a nitride semiconductor and successively disposedin that order towards the top. A ridge 4 a projecting upward is createdin the p-side semiconductor layer 4. In the description herein, thedirection from the n-side semiconductor layer 2 to the p-sidesemiconductor layer 4 is referred to as upward or towards the top, andthe opposite direction is referred to as downward or towards the bottom.

The p-side semiconductor layer 4 has a first part 41, an electronbarrier layer 42, and a second part 43. The first part 41 is disposed incontact with the upper face of the active layer 3 and has at least onesemiconductor layer. The first part 41 is undoped. The electron barrierlayer 42 is disposed in contact with the upper face of the first part41. The electron barrier layer 42 has a larger band gap energy than thefirst part 41 and contains a p-type impurity. The second part 43 isdisposed in contact with the upper face of the electron barrier layer.The second part 43 has at least one p-type semiconductor layercontaining a p-type impurity. The lower end of the ridge 4 a ispositioned in the first part 41. In other words, the ridge 4 a is madeup of a portion of the first part 41, the electron barrier layer 42, andthe second part 43. In the description herein, the term “undoped” refersto a layer or the like that is not intentionally doped. It is fair tosay that a layer is undoped when the impurity concentration does notexceed the detection limit as a result of the analysis by secondary ionmass spectrometry (SIMS). Alternatively, the state in which the impurityconcentration is under 1×10¹⁷/cm³ can be defined as undoped. Forexample, the first part 41 can be called undoped when a p-type or n-typeimpurity concentration is under the detection limit. However, becausethe first part 41 is in contact with the electron barrier layer 42,which has a high p-type impurity concentration, the p-type impuritymight occasionally be detected when analyzed even if it were notintentionally doped with a p-type impurity. In this case, the p-typeimpurity concentration detected may be above 1×10 ¹⁷/cm³ because thep-type impurity is diffused into the first part 41 from the electronbarrier layer 42, but the p-type impurity concentration detected ispreferably under 1×10¹⁸/cm³. In this case, the p-type impurityconcentration detected in the first part 41 may decrease away from theelectron barrier layer 42. Moreover, when the first part 41 or the likeis formed to be undoped, it might turn out to contain an unintendedimpurity such as H, C, or the like. Even in this case, it can still becalled undoped. In the description herein, the film thickness orthickness of a certain layer or part refers to the shortest distancefrom the lowermost face to the uppermost face of the layer or part. Inthe case in which the lowermost face and/or the uppermost face hasindentations and/or projections such as V-pits, the shortest distancebetween the flat parts of the uppermost and lowermost faces whereindentations or projections are absent can be considered as the filmthickness or thickness of the layer or part.

The semiconductor laser element 100 has the structures (1) to (3)described below. (1) The first part 41 has an undoped p-side compositiongraded layer 411 in which the band gap energy increases towards the topand an undoped p-side intermediate layer 412 disposed above the p-sidecomposition graded layer 411. The lower end of the ridge is positionedin the p-side intermediate layer 412. (2) The thickness of the secondpart 43 is smaller than the thickness of the first part 41, and thelower end of the ridge 4 a is positioned in the first part 41. (3) Thethickness of the first part 41 is at least 400 nm, and the lower end ofthe ridge 4 a is positioned in the first part 41. The semiconductorlaser element 100 may satisfy any one of these structures (1) to (3), orsimultaneously satisfy two or more.

The structure (1) will be described first. As shown in FIG. 2A, thep-side composition graded layer 411 is a layer in which the band gapenergy increases towards the top. Having such a layer can enhanceoptical confinement to the active layer 3. The p-side intermediate layer412 is a different layer from the p-side composition graded layer 411.Disposing not only the p-side composition graded layer 411, but also thep-side intermediate layer 412 can provide a relatively larger thicknessto the first part 41. This can keep the peak light intensity at adistance from the electron barrier layer 42 and the second part 43 thatcontain p-type impurities. These features can lower the light intensityin the second part 43, thereby reducing optical absorption losses. Thiscan consequently increase the efficiency of the semiconductor laserelement 100. An example of the efficiency of the semiconductor laserelement 100 is slope efficiency, which is represented by a slope in thecurrent-light output graph at the threshold current value or higher.

The lower end of the ridge 4 a is positioned deeper than the electronbarrier layer 42, i.e., in the p-side intermediate layer 412. This canreduce the distance between the lower end of the ridge 4 a and theactive layer 3 even when the first part 41 is relatively thick.Accordingly, optical confinement in the transverse direction can beenhanced as compared to the case in which the lower end of the ridge 4 ais positioned higher than the first part 41. Weak optical confinementmakes the horizontal transverse mode of the semiconductor laser element100 unstable, which might allow kinks to occur in the I-Lcharacteristics that show the relationship between current and lightoutput. Forming a ridge 4 a with the lower end in the p-sideintermediate layer 412 can enhance optical confinement in the transversedirection and can stabilize the horizontal transverse mode, therebyreducing the likelihood of causing kinks in the I-L characteristics tooccur. Moreover, the lower end of the ridge 4 a might allow forelectrical leakage attributable to etching damage or the like incurredduring the formation of the ridge 4 a. Thus, the lower end of the ridge4 a is preferably positioned not too close to the active layer 3. Thelower end of the ridge 4 a is preferably positioned in the p-sideintermediate layer 412 rather than the p-side composition graded layer411.

The structure (2) will be described next. Because the first part 41 hasa relatively large thickness, similar to (1), the peak light intensitycan be kept at a distance from the p-type impurity-containing layers,and losses attributable to free carrier absorption in the p-typeimpurity-containing layers can be reduced. Thus, the efficiency of thesemiconductor laser element 100, such as slope efficiency, can beimproved. In addition, the second part 43 having a small thickness canreduce the drive voltage, thereby increasing the efficiency of thesemiconductor laser element 100. Providing a thin p-typeimpurity-containing part can reduce the voltage because in a nitridesemiconductor, p-type impurities such as Mg have a lower activation ratethan n-type impurities, and p-type impurity-containing layers have arelatively high resistance. Although undoped, the first part 41 ispositioned between the electron barrier layer 42 and the active layer 3.It thus tends to show the n-type conductivity rather than completeinsulation properties attributable to factors such as an electronoverflow. It is considered that reducing the thickness of the relativelyhigh resistant p-type impurity-containing second part 43 can lower thedrive voltage and achieve the effect of attenuating a drive voltageincrease that can result from increasing the thickness of the undopedfirst part 41. This effect was confirmed by the Test Result 2 forExamples 1 to 3 described below. Similar to (1), the lower end of theridge 4 a is preferably positioned in the first part 41 because thefirst part 41 has a relatively large thickness, which can enhanceoptical confinement in the transverse direction.

Structure (3) will be described next. The first part 41 having arelatively large thickness, i.e., at least 400 nm, can reduce losses inthe p-type impurity-containing layers, thereby increasing the efficiencyas in the cases of (1) and (2). Similar to (1) and (2), positioning thelower end of the ridge 41 in the first part 41 can enhance opticalconfinement in the transverse direction.

With regard to the structure of the first part 41, several patterns wereassumed, and equivalent refractive index simulations were performed. Inthe simulations, the refractive index of each layer was calculated basedon the composition ratio of the nitride semiconductor constituting thelayer by using the formula disclosed by M. J. Bergmann, et al., Journalof Applied Physics, Vol. 84 (1998), pp. 1196-1203. For CalculationExample 1, the structure having only a p-side composition graded layer411 of 260 nm in thickness as the first part 41 was used. ForCalculation Examples 2 to 5, the structure having a p-side compositiongraded layer 411 and a p-side intermediate layer 412 as the first part41 was used. The film thicknesses of the p-side intermediate layer 412in Calculation Examples 2 to 5 were 50 nm, 100 nm, 200 nm, and 400 nm,respectively. In other words, the thicknesses of the first part 41 inCalculation Examples 1 to 5 were 260 nm, 310 nm, 360 nm, 460 nm, and 660nm, respectively. The layer structures of those other than the firstpart 41 were generally the same as those of the semiconductor laserelement 100 of Example 1 described below, with slight differences indetail, including the second n-side optical guide layer 27, which was acomposition graded layer changing from GaN to In_(0.05)Ga_(0.95)N. Thelayer structures of those other than the first part 41 were the sameamong Calculation Examples 1 to 3, and those in Calculation Examples 4and 5 were the same as those in Calculation Examples 1 to 3 except suchthat the thickness of the first n-side optical guide layer 26 wasreduced to two thirds. The thickness of the first n-side optical guidelayer 26 in Calculation Examples 4 and 5 was reduced in order tocompensate for a peak electric field intensity deviation from the activelayer 3 attributable to the thick first part 41.

FIG. 7 shows the relationship between the thickness of the first part 41and the percentage of light leakage to the second part 43 with respectto Calculation Examples 1 to 5. As shown in FIG. 7, the larger thethickness of the first part 41, the smaller the light leakage to thesecond part 43 resulted with the degree of decrease moderating at aroundthe thickness of 400 nm. From this, the thickness of the first part 41is preferably at least 400 nm. This can minimize the light leakage tothe second part 43, for example, under 3%.

Semiconductor Laser Element 100

As shown in FIG. 1, the semiconductor laser element 100 has a substrate1, and disposed on the substrate, an n-side semiconductor layer 2, anactive layer 3, and a p-side semiconductor layer 4. The semiconductorlaser element 100 is an edge emitting laser element having a lightemitting facet and a light reflecting facet that cross the principalfaces of the semiconductor layers such as the active layer 3. A ridge 4a is created in the upper portion of the p-side semiconductor layer 4.The ridge 4 a has a mesa structure. The top view shape of the ridge 4 ais a rectangle that is longer in the direction linking the lightemitting facet and the light reflecting facet, for example, a rectanglehaving short sides paralleling the light reflecting facet and long sidesperpendicular to the light reflecting facet. The part of the activelayer 3 located immediately under the ridge 4 a and in the vicinitythereof is the optical waveguide region. An insulation film 5 can bedisposed on the lateral faces of the ridge 4 a and the surface of thep-side semiconductor layer 4 that continues from the lateral faces ofthe ridge 4 a. The substrate 1 is made of an n-type semiconductor, forexample, and an n-electrode 8 is disposed on the lower face. Moreover, ap-electrode 6 is disposed in contact with the upper face of the ridge 4a, and a p-side pad electrode 7 is further disposed thereon.

The semiconductor laser element 100 can have a structure to emit laserlight in the long wavelength range. The semiconductor laser element 100can emit laser light in the green wavelength range, for example, laserlight having a wavelength of at least 530 nm. In other words, it canemit laser light having a peak wavelength of 530 nm or higher. As theemission wavelength increases from the blue wavelength range to thegreen wavelength range, leakage of light from the optical guide layerincreases due to wavelength dependent refractive index variation, i.e,dispersion. This consequently increases the threshold current toincrease the current density during laser emission. The higher thecurrent density, the larger the effective transition interval becomesdue to screening of localized energy levels and band filling effect,which can shift the emission wavelength to the short-wavelength side.Disposing a p-side composition graded layer 411 can reduce the laseremission threshold current density, thereby restraining the wavelengthfrom shifting to the short-wavelength side as discussed below. In thecase of a semiconductor laser element emitting light in the greenwavelength range, there hardly is a refractive index difference betweenthe clad layer and the active layer due to wavelength dependentrefractive index variation, i.e., dispersion. Thus, it still has ahigher threshold current density and lower slope efficiency than asemiconductor laser element emitting light in the blue wavelength range.Accordingly, a semiconductor laser element in the green wavelength rangeemploying the features of this embodiment is expected to achieve agreater efficiency improving effect resulting from reduced opticalabsorption losses in the p-type semiconductor layers. Optical absorptionlosses in the p-type semiconductor layers can occur regardless of thewavelength of the laser light emitted by the semiconductor laser element100. Accordingly, the wavelength of the laser light emitted by thesemiconductor laser element 100 is not limited to the green wavelengthregion; for example, it may be in the blue wavelength region.

Substrate 1

For the substrate 1, a nitride semiconductor substrate composed of GaN,for example, can be used. Examples of the n-side semiconductor layer 2,the active layer 3, and the p-side semiconductor layer 4 to be grown onthe substrate 1 include semiconductors grown in substantially the c-axisdirection. For example, by using a GaN substrate having a +c plane((0001) plane) as a principal plane, each semiconductor layer can begrown on the +c plane. Having a +c plane as a principal plane herein mayinclude those that are off by about ±1 degree. Employing a substratehaving a +c plane as a principal plane provides the benefit of higherproduction efficiency.

N-Side Semiconductor Layer 2

The n-side semiconductor layer 2 can have a multilayer structurecomposed of nitride semiconductors, such as GaN, InGaN, AlGaN, or thelike. The n-side semiconductor layer 2 includes at least one n-typesemiconductor layer. Examples of n-type semiconductor layers includelayers composed of a nitride semiconductor containing an n-typeimpurity, such as Si, Ge, or the like. The n-side semiconductor layer 2can have an n-side clad layer and an n-side optical guide layer, and mayinclude additional layers. The n-side clad layer has a larger band gapenergy than the n-side optical guide layer. Although not as much as ap-type impurity, an n-type impurity is also a factor for opticalabsorption. Thus, the n-side optical guide layer is preferably undoped,or has a lower n-type impurity concentration than that in the n-sideclad layer.

FIG. 4 shows an example of the layer structure of the n-sidesemiconductor layer 2. The n-side semiconductor layer 2 shown in FIG. 4has, successively from the substrate 1 side, an underlayer 21, a firstn-side clad layer 22, a crack suppressing layer 23, an intermediatelayer 24, a second n-side clad layer 25, a first n-side optical guidelayer 26, a second n-side optical guide layer 27, and a hole blockinglayer 28. The hole blocking layer 28 has a first hole blocking layer 281and a second hole blocking layer 282.

An n-type impurity is doped to the layers from the underlayer 21 to thefirst n-side optical guide layer 26. The underlayer 21, for example, isan n-type AlGaN layer. The first n-side clad layer 22, for example, isan n-type impurity-doped layer having a larger band gap energy than theunderlayer 21. The crack suppressing layer 23, for example, is composedof InGaN, and has a smaller band gap energy than a well layer in theactive layer 3. Disposing a crack suppressing layer 23 can reduce theprobability of the occurrence of cracks. The intermediate layer 24 has alattice constant between those of the crack suppressing layer 23 and thesecond n-side clad layer 25, and is composed of GaN, for example. In thecase in which the crack suppressing layer 23 is an InGaN layer, it ispreferable to grow a GaN intermediate layer 24 before allowing thesecond n-side clad layer 25 to grow. If the second n-side clad layer 25were grown in contact with the upper face of the crack suppressing layer23, a portion of the crack suppressing layer 23 might decompose tooccasionally affect even the growth of the active layer 3. Disposing theintermediate layer 24 can reduce the probability of the occurrence ofsuch decomposition. The intermediate layer 24 has a thickness that issmaller than that of the crack suppressing layer 23, for example. Thesecond n-side clad layer 25 is a layer having a larger band gap energythan the underlayer 21, for example, and may be made of the samematerial as the first n-side clad layer 22. The first n-side clad layer22 and the second n-side clad layer 25 are made of, for example, AlGaN.One or both of the first n-side clad layer 22 and the second n-side cladlayer 25 may have the largest band gap energy in the n-sidesemiconductor layer 2. The composition and/or the n-type impurityconcentration of the first n-side clad layer 22 may be the same as thoseof the second n-side clad layer 25. There may be a single n-side cladlayer, and in this case the crack suppressing layer may be absent, ormay be disposed on or under the n-side clad layer.

The first n-side optical guide layer 26 has a smaller band gap energyand a lower n-type impurity concentration than the first n-side cladlayer 22 and the second n-side clad layer 25. The first n-side opticalguide layer 26 is composed of GaN, for example. The band gap energy ofthe second n-side optical guide layer 27 is larger than that of a welllayer in the active layer 3, but smaller than that of the first n-sideoptical guide layer 26. Because the second n-side optical guide layer 27is positioned closer to the active layer 3 than is the first n-sideoptical guide layer 26, the second n-side optical guide layer 27preferably has a lower n-type impurity concentration than that of thefirst n-side optical guide layer 26 for optical absorption lossreduction purposes. The second n-side optical guide layer 27 is composedof undoped InGaN, for example.

The second n-side optical guide layer 27 may be a composition gradedlayer in which the band gap energy becomes smaller as the distance tothe active layer 3 decreases. In the case of disposing a compositiongraded layer as the n-side optical guide layer, the layer's compositionis changed stepwise such that the refractive index becomes higher as thedistance to the active layer 3 decreases. This can continuously form anoptical waveguide barrier in the n-side composition graded layer,thereby enhancing optical confinement to the active layer 3. For thecriteria used in determining the magnitudes of band gap energy andimpurity concentration of the composition graded layer relative to theother layers, the average value for the composition graded layer can beused. An average value of a composition graded layer refers to the valueobtained by multiplying the band gap energy or the like of each sublayerin the composition graded layer by the thickness, followed by dividingthe sum of the products by the total film thickness. In the case ofdisposing a composition graded layer in the n-side semiconductor layer 2in which the lattice constant increases as the distance to the activelayer 3 decreases, it is preferable to dope an n-type impurity to thecomposition graded layer. The composition graded layer, in other words,is made up of multiple sublayers each having a slightly differentcomposition. For this reason, in the composition graded layer, it isdifficult to avoid the occurrence of fixed charges even if the rate ofcomposition change is reduced. Because the doping of an n-type impuritycan screen fixed charges, the degree of voltage increase attributable tofixed charges can be lowered.

An n-type impurity is preferably contained in at least one portion ofthe hole blocking layer 28. This can increase the hole blockingefficiency. For example, the first hole blocking layer 281 is made ofGaN, and the second hole blocking layer 282 is made of InGaN.

Active Layer 3

The active layer 3 can be have a multilayer structure composed ofnitride semiconductor layers, such as GaN, InGaN, and the like. Theactive layer 3 has a single quantum well structure or multiple quantumwell structure. It is considered that a multiple quantum well structurecan more easily achieve sufficient gain than a single quantum wellstructure. In the case in which the active layer 3 has a multiplequantum well structure, it has multiple well layers, and intermediatebarrier layers interposed between well layers. For example, the activelayer 3 includes, successively from the n-side semiconductor layer 2side, a well layer, an intermediate barrier layer, and a well layer. Ann-side barrier layer 31 may be disposed between the well layer closestto the n-side semiconductor layer 2 and the n-side semiconductor layer2. The n-side barrier layer 31 may be allowed to function as a part ofthe hole blocking layer 28. The hole blocking layer 28 or the n-sideoptical guide layer (second n-side optical guide layer 27) may beallowed to function as a barrier layer while omitting the n-side barrierlayer 31. Similarly, a p-side barrier layer may be disposed between thewell layer closest to the p-side semiconductor layer 4 and the p-sidesemiconductor layer 4. In the case in which no p-side barrier layer isprovided or the p-side barrier layer has a small thickness, a portion ofthe p-side semiconductor layer 4 may be allowed to function as a p-sidebarrier layer. In the case of disposing a p-side barrier layer in theactive layer 3, the film thickness of the p-side barrier layer can be 5nm at most, for example. In other words, the shortest distance betweenthe p-side semiconductor layer 4 and a well layer in the active layer 3is 5 nm at most, for example. As described above, moreover, because thedoping of a p-type impurity increases optical absorption losses, theactive layer 3 is preferably formed without doping any p-type impurity.Each layer in the active layer 3, for example, is an undoped layer.

In the case of a semiconductor laser element having an emissionwavelength of at least 530 nm, the Indium composition ratio x in anIn_(x)Ga_(1-x)N well layer can slightly vary depending on the layerstructures of those other than the active layer 3, but is at least 0.25,for example. The upper limit for the Indium composition ratio x in awell layer, for example, is 0.50 at most. At this time, the emissionwavelength of the semiconductor laser element is believed to be about600 nm at most.

P-Side Semiconductor Layer 4

The p-side semiconductor layer 4 can have a multilayer structurecomposed of nitride semiconductors, such as GaN, InGaN, AlGaN, or thelike. The p-side semiconductor layer 4 can include a p-side clad layerand a p-side optical guide layer, and may include additional layers. Inthe case of disposing a transparent conductive film as a p-electrode 6,this can be allowed to function as a clad layer. Thus, a clad layer doesnot have to be disposed in the p-side semiconductor layer 4.

The p-side semiconductor layer 4 includes at least one p-typesemiconductor layer. Examples of p-type semiconductor layers include anitride semiconductor layer containing a p-type impurity such as Mg orthe like. Because the activation rates of p-type impurities are lowerthan those of n-type impurities such as Si, the free carrier absorptionloss in the p-type semiconductor layer is increased by p-typeimpurities. The larger the absorption loss, the lower the slopeefficiency of the semiconductor laser element 100 results. An internalloss a, generally includes free carrier absorption loss arm Assumingthat amc represents the internal loss other than free carrier absorptionloss arc, the threshold modal gain required for laser emission isrepresented by the model formula below that depends on free carrierabsorption loss. Here, arc, and am represent free carrier absorptionloss, average internal loss, and mirror loss, respectively. As a matterof convenience, the average is used without taking modal distributioninto consideration. In the formula, F represents the optical confinementfactor for the active region, and g_(th) represents the threshold gainfor laser emission.

Γg_(th)=α_(fc)+α_(int)+α_(m)

The free carrier absorption loss here includes the losses in layersother than the active layer 3. In the p-type semiconductor layer, forexample, the free carrier absorption loss can be approximated by theproduct of the p-type impurity concentration n, the factor Gfc thatreflects the free carrier absorption cross-sectional area, and theaverage light leakage to the p-type semiconductor layer Γ_(p). In otherwords, for a certain impurity concentration in a p-type semiconductorlayer, the free carrier absorption loss α_(fc) increases as the lightleakage to the p-type semiconductor layer increases. Similarly, for acertain light leakage to the p-type semiconductor layer, the freecarrier absorption loss α_(fc) increases as the impurity concentrationin the p-type semiconductor layer increases. Because a low p-typeimpurity concentration raises a concern of considerably increasing thedrive voltage, it is particularly effective to reduce light leakage to alayer of high p-type impurity concentration for free carrier absorptionloss afc reduction purposes.

α_(fc) =n×σ _(fc)×Γ_(p)

It can be understood from the above formulas that an increase in lightleakage to the p-type clad layer increases the free carrier absorptionloss to thereby increase the threshold gain g_(th). During laseremission, a steady state of g=g_(th) is maintained in the resonator. Insuch a steady state, the modal gain depends monotonically on the carrierdensity. Thus, the carrier density at the laser emission thresholdcurrent or higher is maintained at the threshold carrier density N_(th).The higher the injected carrier density, the more localized energy levelscreening results and the larger the substantial band gap becomes, whichreadily shifts the laser emission wavelength to the short-wavelengthside. Reducing the free carrier absorption loss afc, thereby reachingthe threshold gain gth at a lower current can reduce both the thresholdcurrent density jth and the threshold carrier density N_(th). This canreduce the injected carrier density, thereby reducing localized levelscreening and allowing for laser emission at a longer wavelength.Accordingly, it is preferable from this perspective as well to reducethe free carrier absorption loss in a semiconductor laser element 100capable of emitting laser light of a long wavelength such as 530 nm orhigher. Even in the case of a semiconductor laser element on the shorterwavelength side, this has the benefit of achieving a laser light sourcehaving a low threshold current.

Here, screening of localized levels being reduced has been explained,but band filling effect can similarly be reduced. Band filling effect,which expands the effective transition interval as a quasi-Fermi levelis separated from the band end by current injection, can also cause ashift to shorter wavelengths. This can also be reduced when thethreshold carrier density is lowered by reducing the free carrierabsorption loss afc.

First Part 41

The first part 41 is a portion of the p-side semiconductor layer 4 thatconnects the active layer 3 and the p-type impurity-containing layers.The first part 41 is the portion that does not include a p-typesemiconductor layer. A p-type impurity-containing layer may be includedin a portion of the first part 41 so long as the layer has a filmthickness and p-type impurity concentration that would not affect thefree carrier absorption loss. In the case of doping with Mg required forp-type conversion, the p-type impurity of at least about 1×10¹⁸/cm³ isrequired, and this highly likely increases the free carrier absorptionloss. Accordingly, the first part 41 is preferably a part that does notinclude a p-type semiconductor layer. The p-type impurity concentrationthroughout the first part 41 is preferably low enough to be undetectablein a SIMS analysis or the like. For example, the first part 41 is formedduring manufacturing without intentionally doping a p-type impurity inits entirety. As described above, the larger the thickness of the firstpart 41, the more reduction in the light leakage to the second part 43results. Thus, the thickness of the first part 41 is preferably at least400 nm. The upper limit value for the thickness of the first part 41 canbe set to one that does not interfere with the supply of holes from thesecond part 43. As shown by the Test Result 3 described below, thelarger the thickness of the first part 41, the more electron overflowresults. From this perspective, the thickness of the first part 41 ispreferably small. The thickness of the first part 41 can be set, forexample, to 660 nm at most. Because of the band gap difference from theelectron barrier layer 42, the first part 41 can reduce the probabilityof the occurrence of an electron overflow. Accordingly, the first part41 preferably has a layer having a smaller band gap energy than theelectron barrier layer 42 as a layer in contact with the electronbarrier layer 42.

In the case of making the first part 41 a low-concentration doped partrather than an undoped part, the p-type impurity concentrationthroughout the first part 41 is preferably lower than the p-typeimpurity concentration of the electron barrier layer 42, more preferablylower than those of both the electron barrier layer 42 and the secondpart 43. An example of the n-type impurity concentration of the firstpart 41 is 2×10¹⁸/cm³. Preferably, the n-type impurity concentration ofthe first part 41 is low enough to be undetectable by a SIMS analysis(i.e., a background level). In other word, the first part 41 preferablycontains substantially no n-type impurity.

P-Side Composition Graded Layer 411 and P-Side Intermediate Layer 412

The first part 41, as shown in FIG. 2A, can have a p-side compositiongraded layer 411 and a p-side intermediate layer 412. The p-sideintermediate layer 412 is disposed above the p-side composition gradedlayer 411. The p-side intermediate layer 412 may be disposed in contactwith the upper face of the p-side composition graded layer 411, or incontact with the lower face of the electron barrier layer 42. Althoughthe structure of the first part 41 is not limited to one having both thep-side composition graded layer 411 and the p-side intermediate layer412, as described above, having a p-side composition graded layer 411can enhance optical confinement to the active layer 3, and having ap-side intermediate layer 412 can make the thickness of the first part41 even larger. Enhancing optical confinement to the active layer 3 bydisposing the p-side composition graded layer 411 can reduce the laseremission threshold current density. This can reduce screening oflocalized energy levels to restrain the emission wavelength fromshifting to short wavelengths caused by an increase in the injectedcurrent. This thus is beneficial in making the emission wavelengthlonger.

The p-side composition graded layer 411 is a layer in which the band gapenergy increases towards the top. The p-side composition graded layer411 has an upper face and a lower face, and the band gap energyincreases from the lower face to the upper face. The band gap energy onthe lower face side is smaller than on the upper face side. Althoughshown as a slope in FIG. 2A, as described below, the p-side compositiongraded layer 411 is a collection of multiple sublayers that aredifferent in composition. Thus, it can be said that the band gap energyin the p-side composition graded layer 411 increases stepwise from thelower face to the upper face. An n-side composition graded layer that ispaired with the p-side composition graded layer 411 may be disposed inthe n-side semiconductor layer 2. An example of such an n-sidecomposition graded layer is a layer in which the band gap energydecreases towards the active layer 3. For example, the p-sidecomposition graded layer 411 and the n-side composition graded layer aresymmetrically formed while interposing the active layer 3. Disposingcomposition graded layers on both sides of the active layer 3 in thismanner can confine light to the active layer 3 from both sides in awell-balanced manner. For the purpose of enhancing the light confinementeffect of the p-side composition graded layer 411, the p-sidecomposition graded layer 411 is preferably disposed near the activelayer 3. Accordingly, the p-side composition graded layer 411 ispreferably disposed in contact with the active layer 3. The shortestdistance between the p-side composition graded layer 411 and the welllayer 32 in the active layer 3 is preferably 5 nm at most.

The p-side composition graded layer 411, for example, functions as ap-side optical guide layer. The film thickness of the p-side compositiongraded layer 411 is larger than the film thickness of the well layer 32,and in the case in which the p-side barrier layer 34 is present, islarger than the film thickness of the p-side barrier layer 34. For thepurpose of enhancing the light confinement effect, the film thickness ofthe p-side composition graded layer 411 is preferably at least 200 nm.The film thickness of the p-side composition graded layer 411 can be setto 500 nm at most, preferably 350 nm at most, more preferably 300 nm atmost. The band gap energy at the lower end of the p-side compositiongraded layer 411, in the case of disposing a p-side barrier layer 34, ispreferably smaller than the band gap energy of the p-side barrier layer34. The band gap energy at the upper end of the p-side compositiongraded layer 411 may be equal to or larger than the band gap energy ofthe p-side barrier layer 34. The p-side composition graded layer 411preferably has the structure such that the refractive indexmonotonically decreases, and the band gap energy monotonicallyincreases, from the active layer 3 side to the electron barrier layer 42side in order to reduce an electron overflow while drawing light to theactive layer 3.

The p-side composition graded layer 411, as shown in FIG. 5, can bedescribed as being made up of a plurality of sublayers 411 a, 411 b, 411c, 411 y, and 411 z each having a different composition from oneanother. FIG. 5 is a partially enlarged view of the p-side compositiongraded layer 411 and the vicinity, and a number of sublayers other thanthose explicitly shown are present between the sublayers 411 c and 411y. In the case of forming the p-side composition graded layer 411 withInGaN or GaN, the lowermost sublayer 411 a of the p-side compositiongraded layer 411 is made of In_(a)Ga_(1-a)N (0<a<1), and the uppermostsublayer 411 z of the p-side composition graded layer 411 is made ofIn_(z)Ga_(1-z)N (0≤z<a). The upper limit value of the Indium compositionratio a is, for example, 0.25. Considering the reduction ofcrystallinity degradation, the Indium composition ratio a is preferably0.1 at most. Moreover, the lattice constant difference between twoadjacent sublayers is preferably small. This can reduce distortion. Forthis purpose, the p-side composition graded layer 411 is preferablyformed by gradually changing the composition of sublayers in smallthickness. Specifically, the p-side composition graded layer 411preferably has an Indium composition ratio that decreases per filmthickness of 25 nm at most from the lower face to the upper face. Inother words, the film thickness of each of the sublayers 411 a, 411 b,411 c, 411 y, and 411 z is preferably 25 nm at most. The film thicknessof each of the sublayers 411 a, 411 b, 411 c, 411 y, and 411 z is morepreferably 20 nm at most. The lower limit value of the film thickness ofeach of the sublayers 411 a, 411 b, 411 c, 411 y, and 411 z is, forexample, about one monoatomic layer (about 0.25 nm). The Indiumcomposition ratio difference between two adjacent sublayers (e.g., thesublayer 411 a and the sublayer 411 b) is preferably 0.005 at most, morepreferably 0.001 at most. The lower limit value is, for example, about0.00007.

Such a range is preferably satisfied throughout the p-side compositiongraded layer 411. In other words, all sublayers preferably satisfy sucha range. In a p-side composition graded layer 411 of 260 nm in filmthickness, for example, when the lowermost sublayer 411 a isIn_(0.05)Ga_(0.95)N and the uppermost sublayer 411 z is GaN, thesublayers are grown under the manufacturing conditions that graduallychange the composition in 120 steps. The number of composition changesmade in the p-side composition graded layer 411 is preferably at least90 times. The composition change rate (i.e., the composition differencebetween two adjacent sublayers) in the p-side composition graded layer411 may remain constant or vary across the entire p-side compositiongraded layer 411. The composition change rate in the p-side compositiongraded layer 411 is preferably 0.001 at most across the entire layer. Inthe case of disposing an n-side composition graded layer, ranges for thecomposition, composition change rate, and film thickness similar to thepreferable ranges for the p-side composition graded layer 411 can beemployed.

It is preferable not to position the lower end of the ridge 4 a in thep-side composition graded layer 411. If the lower edge of the ridge 4 ais positioned in the p-side composition graded layer 411, the effectiverefractive index difference between inside and outside of the ridge 4 awould vary considerably depending on the depth. The degree of suchvariation in the effective reactive index difference between inside andoutside of the ridge 4 a can be reduced by positioning the lower edge ofthe ridge 4 a in a single composition layer. Accordingly, in the case ofdisposing a p-side composition graded layer 411, it is preferable todispose a single composition layer for the purpose of positioning thelower edge 4 a of the ridge. The single composition layer preferably hasa thickness larger than the depth variation of the ridge 4 a resultingfrom forming the ridge 4 a. In this manner, even when the depth of theridge 4 a varies, the lower end of the ridge 4 a can be positioned inthe single composition layer, thereby reducing the variation in theeffective refractive index difference between inside and outside of theridge 4 a. The film thickness of such a single composition layer forpositioning the lower edge of the ridge 4 a is preferably larger than asublayer of the composition graded sublayer, and can be larger than 25nm, for example. The film thickness of the single composition layer canbe set to, for example, 600 nm at most. A single composition layerrefers to a layer formed by not intentionally changing the composition.

The p-side intermediate layer 412 may be such a single compositionlayer, or may have a multilayer structure. In the case in which thep-side intermediate layer 412 has a multilayer structure, at least thelayer in which the lower end of the ridge 4 a will be positioned amongthe layers making up the p-side intermediate layer 412 can be formed asa single composition layer. In the case in which the p-side intermediatelayer 412 has a multilayer structure, as shown in FIG. 2B, the p-sideintermediate layer 412 can have a first layer 412A and a second layer412B. The first layer 412A has a band gap energy larger than the averageband gap energy of the p-side composition graded layer 411, but smallerthan the band gap energy of the electron barrier layer 42. The secondlayer 412B has a band gap energy larger than the band gap energy of thefirst layer 412A, but smaller than the band gap energy of the electronbarrier layer 42. The first layer 412A and the second layer 412B areundoped. The refractive indices of these layers can be set in descendingorder with the average refractive index of the p-side composition gradedlayer 411 being highest, followed by the refractive index of the firstlayer 412A, and the refractive index of the second layer 412B. In thedescription herein, the average band gap energy refers to that obtainedby multiplying each layer's band gap energy by the film thickness anddividing the sum of the products by the total film thickness. In thecase of a composition graded layer, the average band gap energy isobtained by multiplying each sublayer's band gap energy by the filmthickness, and dividing the sum of the products by the total filmthickness. Similar calculations can be performed to obtain the averagerefractive index and the average composition ratio.

Disposing a second layer 412B can reduce light leakage to the secondpart 43, thereby reducing the free carrier absorption loss occurring inthe second part 43. Including a second layer 412B having a lowerrefractive index than that of the first layer 412A in the p-sideintermediate layer 412 allows the p-side intermediate layer 412 to havea smaller film thickness to achieve the same degree of opticalconfinement effect than a p-side intermediate layer 412 having a firstlayer 412A alone. As described above, the first part 41 preferably has alarge thickness in order to reduce light leakage to the second part 43.On the other hand, it is effective for the first part 41 to have a smallthickness in order to further reduce the voltage. Disposing a p-sidecomposition graded layer 411 and a second layer 412B can reduce lightleakage to the second part 43 while restraining the voltage fromincreasing.

In the case in which both the first layer 412A and the second layer 412Bare single composition layers, the lower end of the ridge 4 a ispreferably positioned in either the first layer 412A or the second layer412B. This can reduce the variation in the effective refractive indexdifference between inside and outside of the ridge 4 a even when thelower end position of the ridge 4 a varied during manufacturing asdescribed above. The lower end of the ridge 4 a may be positioned in thefirst layer 412A, as shown in FIG. 2B and FIG. 2C, or in the secondlayer 412B, as shown in FIG. 2D. The first layer 412A is, for example, aGaN layer. The second layer 412B is, for example, an AlGaN layer. Inthis case, the aluminum composition ratio of the second layer 412B canbe set to at least 0.01% and 10% at most, for example. The filmthickness of the second layer 412B can be set to at least 1 nm and 600nm at most. In the case in which the refractive index of the secondlayer 412B is smaller than that of the first layer 412A, the filmthickness of the second layer 412B is preferably larger than that of thefirst layer 412A. This can further enhance the effect of confining lightto the active layer 3. For example, the second layer 412B is set to havea film thickness larger than that of the first layer 412A by at least 50nm. In this case, moreover, because the first layer 412A has a smallerimpact on optical confinement than the p-side composition graded layer411 and the second layer 412B, reducing the thickness of the first layer412A can reduce electrons overflowing from the active layer 3 whilereducing the light leakage to the second part 43. This can improve theslope efficiency of the semiconductor laser element. From thisperspective, the film thickness of the first layer 412A is preferably100 nm at most, more preferably 50 nm at most. The film thickness of thefirst layer 412A, furthermore, is preferably set to one half of thethickness of the second layer 412B at most, more preferably one quarterat most. The film thickness of the first layer 412A can be set to atleast 1 nm.

The second layer 412B, as shown in FIG. 2B, may have a larger band gapenergy than the band gap energy of the layer in the second part 43 thatis in contact with the electron barrier layer 42 (the lower p-typesemiconductor layer 431 in FIG. 2B). Disposing such a layer having arelatively large band gap energy in the first part 41 is consideredbeneficial in reducing light leakage to the second part 43 that has alarger absorption loss than the first part 41. In this case, it ispreferable to employ a material for the p-electrode 6 that can functionas a clad layer. This eliminates the need for disposing a p-side cladlayer in the second part 43, thereby reducing the bias voltage to beapplied. This will be explained in detail below.

In the case of allowing the p-electrode 6 to function as a clad layer, alight transmissive material is used for the p-electrode 6, which cancause absorption loss to occur. For this reason, if there is much lightleakage to the p-electrode 6 that needs to be reduced, a layerfunctioning as a p-type clad layer such as a p-type layer containing Alcan be disposed in the second part 43. It is preferable for the aluminumcomposition ratio to be relatively high in order for the layer tofunction as a p-type clad layer, but the higher the aluminum compositionratio, the higher the activation energy for activating the p-typeimpurity in the layer will be required. Insufficient p-type conversionof the second part 43 increases the series resistance to increase thebias voltage to be applied. Accordingly, in the case of disposing ap-type clad layer in the second part 43, the acceptor concentration isincreased by increasing the doped amount of the p-type impurity, forexample. However, as described above, increasing the doped amount of ap-type impurity increases optical absorption losses and reduces opticaloutput. In the case of a structure where an undoped AlGaN layer isdisposed between the active layer 3 and the electron barrier layer 42shown in FIG. 2B, the voltage hardly increases even if the aluminumcomposition ratio is increased because p-type conversion of the AlGaNlayer is unnecessary. Moreover, in the case of an undoped AlGaN layer,the optical absorption loss hardly increases. An undoped layer generallyhas high resistance, and disposing such a layer tends to increasevoltage, but disposing an undoped AlGaN layer in the first part 41differs from the general trend. This is believed to be because donorsfunction as the majority carriers and acceptors, requiring largeractivation energy than donors, function as the minority carriers in theundoped AlGaN layer disposed between the active layer 3 and the electronbarrier layer 42 when applying a bias voltage. Accordingly, an AlGaNlayer having a relatively high aluminum composition ratio similar to thesecond layer 412B can be disposed in the first part 41. In other words,a layer having a large band gap energy can be disposed in the first part41. Disposing such a layer can reduce the light leakage to the secondpart 43, which eliminates the need to dispose a p-type clad layer in thesecond part 43. That is, the aluminum composition ratio of the secondpart 43 can be reduced. This can reduce the series resistance of thesecond part 43, thereby lowering the voltage for the semiconductor laserelement 100.

FIG. 3A to FIG. 3C schematically show examples of the relative band gapenergy of the uppermost layer of the first part 41, the electron barrierlayer 42, and the lowermost layer of the second part 43. The uppermostlayer of the first part 41 is in contact with the lower face of theelectron barrier layer 42, and the lowermost layer of the second part 43is in contact with the upper face of the electron barrier layer 42. InFIG. 3A, the lowermost layer's band gap energy is smaller than theuppermost layer's band gap energy. In FIG. 3B, the lowermost layer'sband gap energy is equal to the uppermost layer's band gap energy. InFIG. 3C, the lowermost layer's band gap energy is larger than theuppermost layer's band gap energy. For the reasons stated above, asshown in FIG. 3A, the band gap energy of the lowermost layer of thesecond part 43 (e.g., the lower p-type semiconductor layer 431) ispreferably smaller than the band gap energy of the uppermost layer ofthe first part 41 (e.g., the second layer 412B). This can reduce lightleakage to the second part 43, and is suited for the structure thatemploys for the p-electrode 6 a material that functions as a clad layer.In the case of this construction, it is possible to lower the biasvoltage to be applied to the semiconductor laser element 100. Withregard to band gap energy magnitude relation, in the case in which theuppermost layer and/or the lowermost layer do not have constant band gapenergy, such as a superlattice layer or composition graded layer, themagnitudes can be compared by using the average band gap energy. In thecase of a superlattice layer, the average band gap energy of thesuperlattice layer is obtained by multiplying the band gap energy ofeach sublayer in the superlattice layer by the film thickness, followedby dividing the sum of the products by the total film thickness. In thecase in which the uppermost and lowermost layers are AlGaN layers, themagnitude relation between band gap energy levels can otherwise bestated in terms of magnitude relation between Al ratios.

The lowermost layer can be an AlGaN layer having an aluminum compositionratio of 4% at most. The lowermost layer may be a layer havingessentially zero aluminum composition ratio, i.e., a GaN layer. Thelowermost layer, for example, can be a p-type semiconductor layercontaining a p-type impurity such as Mg. The lowermost layer may be aquaternary semiconductor layer such as AlInGaN. The second part 43 thatincludes the lowermost layer preferably has an average aluminumcomposition ratio of 4% at most for the purpose of reducing voltage. Theuppermost layer disposed in the first part 41 preferably has an Alcontent of at least 0.01% in part or in whole. More preferably, theaverage aluminum composition ratio of the uppermost layer is set higherthan 4%. Moreover, the uppermost layer may have a larger band gap energythan that of any of the layers (may be a single layer) that make up thesecond part 43. The uppermost layer may be a superlattice layer or acomposition graded layer that includes AlGaN or AlInGaN. The one or morelayers connecting the uppermost layer and the active layer 3 can each beformed as a layer having a smaller band gap energy than the band gapenergy of the uppermost layer. Such a layer or layers are, for example,the first layer 412A and the p-side composition graded layer 411 shownin FIG. 2B, but may be different from these layers.

A semiconductor laser element 100 may be constructed as described below.It has an n-side semiconductor layer 2, an active layer 3, and a p-sidesemiconductor layer 4, each made of a nitride semiconductor andsuccessively disposed towards the top, and a ridge 4 a projecting upwardcreated in the p-side semiconductor layer 4. The p-side semiconductorlayer 4 has: an undoped first part 41 disposed in contact with the upperface of the active layer 3 and having at least one semiconductor layer;an electron barrier layer 42 containing a p-type impurity disposed incontact with the upper face of the first part 41 and having a largerband gap energy than the first part 41; and a second part 43 disposed incontact with the upper face of the electron barrier layer 42 and havingat least one p-type semiconductor layer containing a p-type impurity.The first part 41 has the uppermost layer in contact with the lower faceof the electron barrier layer 42, and the second part 43 has thelowermost layer in contact with the upper face of the electron barrierlayer 42, wherein the band gap energy of the lowermost layer is smallerthan the band gap energy of the uppermost layer. The lower end of theridge 4 a is positioned in the first part 41.

Electron Barrier Layer 42

The electron barrier layer 42 contains a p-type impurity such as Mg orthe like. The band gap energy of the electron barrier layer 42 is largerthan the band gap energy of the first part 41. In the case in which thefirst part 41 is of a multilayer structure as described above, theelectron barrier layer 42 is formed as a layer having a larger band gapenergy than any of the layers making up the first part 41. The electronbarrier layer 42 having a larger band gap energy as described aboveallows the electron barrier layer 42 to function as a barrier to theelectrons overflowing from the active layer 3. The electron barrierlayer 42 preferably has a band gap energy difference of at least 0.1 eVfrom the uppermost layer of the first part 41. The band gap energydifference between the two can be set, for example, to 1 eV at most. Theelectron barrier layer 42, for example, is formed as a layer having thehighest band gap energy among all in the p-side semiconductor layer 4.The electron barrier layer 42 may be a layer having a smaller filmthickness than the p-side composition graded layer 411. The electronbarrier layer 42 may have a multilayer structure. In this case, theelectron barrier layer 42 has a layer that has a larger band gap energythan any of the layers making up the first part 41. For example, asshown in FIG. 2A and others, it may have a first electron barrier layer42A and a second electron barrier layer 42B. In the case in which asuperlattice layer is included in the first part 41 or the electronbarrier layer 42, the magnitudes are compared using the average band gapenergy of the superlattice layer rather than the band gap energy of eachlayer making up the superlattice layer. The electron barrier layer 42 ismade of AlGaN, for example. In the case of forming an electron barrierlayer 42 with AlGaN, the aluminum composition ratio may be set in arange of 8 to 30%. The film thickness of the electron barrier layer 42can be set, for example, to at least 5 nm, and 100 nm at most.

As shown in FIG. 2C and FIG. 2D, the shortest distance from the lowerend of the ridge 4 a to the electron barrier layer 42 is preferablylarger than the shortest distance from the upper face of the ridge 4 ato the electron barrier layer 42. Such an arrangement can keep the peaklight intensity at a distance from the p-type impurity-containing partssuch as the electron barrier layer 42 and the like, and can reduce thedistance between the lower end of the ridge 4 a and the active layer 3.The shortest distance from the lower end of the ridge to the electronbarrier layer 42, in a cross-sectional view such as FIG. 1, refers tothe shortest distance from a virtual straight line connecting the loweredges of ridge 4 a to the lower face of the electron barrier layer 42.In other words, the electron barrier layer 42 can be said to be locatedcloser to the top of the ridge 4 a. Assuming that the shortest distancefrom the lower end of the ridge 4 a to the active layer 3 is about 436nm, for example, kinks might occur in the I-L characteristics curve.Accordingly, the shortest distance from the lower end of the ridge 4 ato the active layer 3 is preferably 430 nm at most. This can enhance theoptical confinement in the transverse direction.

Second Part 43

The second part 43 has at least one p-type semiconductor layercontaining a p-type impurity. The concentration of the p-type impurityin a p-type semiconductor layer included in the second part 43 can beset to at least 1×10¹⁸/cm³ and 1×10²²/cm³ at most, for example. Becausedrive voltage can be reduced by forming the second part 43 with a smallthickness as described above, the thickness of the second part 43 ispreferably 260 nm at most. The thickness of the second part 43 can beset to at least 10 nm. The second part 43 may include an undoped layer,but the presence of an undoped layer in the second part 43 increases theresistance of the second part 43. Thus, it is preferable that a p-typeimpurity is contained throughout the second part 43. In the case of asuperlattice layer, the average p-type impurity concentration can beconsidered as the p-type impurity concentration of the superlatticelayer. Thus, in the case in which the second part 43 includes asuperlattice layer, the superlattice layer may have a structure in whichan undoped layer and a p-type impurity-containing layer are stacked.

The second part 43 can have an upper p-type semiconductor layer 432 anda lower p-type semiconductor layer 431 as shown in FIG. 2A. The upperp-type semiconductor layer 432 constitutes the upper face of the ridge 4a. In other words, the upper p-type semiconductor layer 432 is theuppermost layer of the second part 43, i.e., the uppermost layer of theridge 4 a. The upper p-type semiconductor layer 432 functions as ap-side contact layer. The lower p-type semiconductor layer 431 isdisposed between the upper p-type semiconductor layer 432 and theelectron barrier layer 42, and has a larger band gap energy than theupper p-type semiconductor layer 432.

The lower p-type semiconductor layer 431 is made of, for example, AlGaN.The upper p-type semiconductor layer 432 is made of, for example, GaN.The lower p-type semiconductor layer 431 preferably has a band gapenergy between that of the electron barrier layer 42 and the upperp-type semiconductor layer 432. Since AlGaN containing a p-type impuritytends to have a higher resistance than GaN containing a p-type impurity,the upper p-type semiconductor layer 432 is preferably a GaN layer towhich a p-type impurity is doped. Disposing an AlGaN lower p-typesemiconductor layer under the upper p-type semiconductor layer 432 canenhance the optical confinement to the active layer 3 as compared to thecase in which the second part 43 is formed with a GaN layer alone. Analuminum composition ratio of the lower p-type semiconductor layer 431being lower than the aluminum composition ratio of the electron barrierlayer 42 can make the resistance of the lower p-type semiconductor layer431 lower than that of the electron barrier layer 42. The lower p-typesemiconductor layer 431 may be allowed to function as a p-side cladlayer. The lower p-type semiconductor layer 431 may be formed as ap-type GaN layer, which can further reduce the resistance of the secondpart 43. In this case, it is preferable to form the p-electrode with amaterial such as ITO to allow it to function as a clad layer.

The film thickness of the upper p-type semiconductor layer 432 can beset to 5 to 30 nm, for example. The film thickness of the lower p-typesemiconductor layer 431 can be set to 1 to 260 nm, for example. The filmthickness of the lower p-type semiconductor layer 431 can be smallerthan the film thickness of the p-side intermediate layer 412, and canfurther be smaller than the film thickness of the second layer 412B. Thelower p-type semiconductor layer 431 and the second layer 412B may bothbe AlGaN layers, and they may have the same aluminum composition ratio.The lower p-type semiconductor layer 431 has a larger film thicknessthan, for example, the electron barrier layer 42. Accordingly, in orderto reduce free carrier absorption losses, the p-type impurityconcentration of the lower p-type semiconductor layer 431 is preferablylower than the p-type impurity concentration of the electron barrierlayer 42. Insulation Film 5, N-electrode 8, P-electrode 6, and P-sidePad Electrode 7

The insulation film 5 can be formed as a single layer film or multilayerfilm made of, for example, an oxide, nitride, or the like of Si, Al, Zr,Ti, Nb, Ta, or the like. The n-electrode 8 is disposed, for example,across the entire lower face of the n-type substrate 1. The p-electrode6 is disposed on the upper face of the ridge 4 a. In the case in whichthe width of the p-electrode 6 is small, a p-side pad electrode 7 havinga larger width than the p-electrode 6 can be formed on the p-electrode 6and a wire or the like can be connected to the p-side pad electrode 7.Examples of materials employed for each electrode include a single layerfilm or multilayer film of metals such as Ni, Rh, Cr, Au, W, Pt, Ti, Al,their alloys, and a conductive oxide containing at least one selectedfrom Zn, In, and Sn. Examples of conductive oxides include ITO (indiumtin oxide), IZO (indium zinc oxide), GZO (gallium-doped zinc oxide), andthe like. The electrodes can be of any thickness to function aselectrodes for a semiconductor element, for example, in a range of about0.1 μm to about 2 μm.

The p-electrode 6 is preferably a transparent conductive film having asmaller refractive index than the refractive index of the active layer3. This allows the p-electrode 6 to function as a clad layer.Furthermore, the p-electrode 6 is preferably a transparent conductivefilm having a smaller refractive index than the refractive index of thesecond part 43. This can further enhance the optical confinement effect.In the case of disposing a p-side clad layer in the second part 43, ap-type impurity-doped AlGaN layer having a relatively high aluminumcomposition ratio, for example, is formed as the p-side clad layer.However, the resistance increases as the aluminum composition ratioincreases. Allowing the p-electrode 6 to function as a clad layer caneliminate the need to dispose a p-side clad layer in the second part 43,or even in the case of disposing a p-side clad layer, the aluminumcomposition ratio thereof can be reduced. This can reduce theresistance, thereby reducing the drive voltage for the semiconductorlaser element 100. An example of a p-electrode 6 that functions as aclad layer is a p-electrode 6 made of ITO.

Manufacturing Method

A method for manufacturing a semiconductor laser element 100 accordingto one embodiment can include the processes S101 to S106 shown in theflowchart in FIG. 6A. In the process S101, an n-side semiconductor layer2 is formed on a substrate 1. In the process S102, an active layer 3 isformed on the n-side semiconductor layer 2. In the process S103, a firstpart 41 having at least one semiconductor layer is formed undoped on theupper face of the active layer 3. In the process S104, an electronbarrier layer 42 having a larger band gap energy than that of the firstpart 41 is formed by doping with a p-type impurity on the upper face ofthe first part 41. In the process S105, a second part 43 having at leastone p-type semiconductor layer doped with a p-type impurity is formed onthe upper face of the electron barrier layer 42. In the process S106, aridge 4 a projecting upward is formed by partially removing the p-sidesemiconductor layer 4 that includes the first part 41, the electronbarrier layer 42, and the second part 43. By following these processes,a semiconductor laser element 100 having an n-side semiconductor layer2, an active layer 3, and a p-side semiconductor layer 4 successivelydisposed towards the top, and a ridge 4 a projecting upward created inthe p-side semiconductor layer, can be produced. The operation, effect,preferable composition, and the like of each layer obtained by eachprocess are as described above. For example, as shown in FIG. 6B, theprocess S103 of forming a first part 41 can include a process S103A offorming an undoped p-side composition graded layer 411 in which the bandgap energy increases towards the top, and a process S103B of forming anundoped p-side intermediate layer 412 above the p-side compositiongraded layer 411. In this case, in the process S106 of forming a ridge 4a, the p-side semiconductor layer 4 can be partially removed such thatthe lower edge of the ridge 4 a is positioned in the p-side intermediatelayer 412. Moreover, in the process S105 of forming a second part 43, asecond part 43 having a smaller thickness than that of the first part 41can be formed. In this case, in the process S106 of forming a ridge 4 a,the p-side semiconductor layer 4 can be partially removed such that thelower end of the ridge 4 a is positioned in the first part 41.Furthermore, in the process S105 of forming a second part 43, a layerhaving a smaller band gap energy than that of the uppermost layer of thefirst part 41 may be formed as the lowermost layer of the second part43.

EXAMPLE 1

A semiconductor laser element having the p-side semiconductor layer 4shown in FIG. 2A and FIG. 4 was prepared as Example 1. An MOCVD systemwas used to prepare an epitaxial wafer for the semiconductor laserelements. For the raw materials, trimethylgallium (TMG), triethylgallium(TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH₃),silane gas, and bis(cyclopentadienyl)magnesium (Cp₂Mg) were used asappropriate.

On an n-type GaN substrate having a +C plane as the upper face(substrate 1), an n-side semiconductor layer 2, an active layer 3, and ap-side semiconductor layer 4 were grown.

First, for the n-side semiconductor layer 2, a Si-dopedAl_(0.018)Ga_(0.982)N layer of 1.0 μm in thickness (underlayer 21), aSi-doped Al_(0.08)Ga_(0.92)N layer of 250 nm in thickness (first n-sideclad layer 22), a Si-doped Al_(0.04)Ga_(0.96)N layer of 150 nm inthickness (crack suppressing layer 23), a Si-doped GaN layer of 10 nm inthickness (intermediate layer 24), a Si-doped Al_(0.08)Ga_(0.92)N layerof 650 nm in thickness (second n-side clad layer 25), a Si-doped GaNlayer of 200 nm in thickness (first n-side optical guide layer 26), anundoped In_(0.03)Ga_(0.97)N layer of 260 nm in thickness (second n-sideoptical guide layer 27), a Si-doped GaN layer of 1.2 nm in thickness(first hole blocking layer 281), and a Si-doped In_(0.05)Ga_(0.95)Nlayer of 4 nm in thickness (second hole blocking layer 282) were grownin that order.

Next, an active layer 3 was grown that included a Si-doped GaN layer(n-side barrier layer 31), an undoped In_(0.25)Ga_(0.75)N layer (welllayer 32), an undoped GaN layer (intermediate barrier layer), an undopedIn_(0.25)Ga_(0.75)N layer (well layer 32), and an undoped GaN layer(p-side barrier layer 34) in that order.

Next, as the p-side semiconductor layer 4, an undoped composition gradedlayer of 260 nm in thickness (p-side composition graded layer 411), anundoped GaN layer of 200 nm in thickness (p-side intermediate layer412), a Mg-doped Al_(0.10)Ga_(0.90)N layer of 3.9 nm in thickness (firstelectron barrier layer 42A), a Mg-doped Al_(0.16)Ga_(0.84)N layer of 7nm in thickness (second electron barrier layer 42B), a Mg-dopedAl_(0.04)Ga_(0.96)N layer of 300 nm in thickness (lower p-typesemiconductor layer 431), and a Mg-doped GaN layer of 15 nm in thickness(upper p-type semiconductor layer 432) were grown in that order.

The p-side composition graded layer 411 was grown by substantiallymonotonically reducing the Indium composition ratio in 120 steps suchthat the composition slope is substantially a straight line startingfrom In_(0.05)Ga_(0.95)N and ending with GaN.

Then the epitaxial wafer having the layers described above was removedfrom the MOCVD system, and a ridge 4 a, an insulation film 5, ap-electrode 6, a p-side pad electrode 7, and an n-electrode 8 wereformed. Then a reflecting film was formed on the light emitting facetand the light reflecting facet, and the wafer was divided intoindividual semiconductor laser elements 100. The depth of the ridge 4 awas set to about 340 nm. In other words, the ridge 4 a was formed sothat the lower end was positioned in the first layer 412A. For thep-electrode 6, an ITO film of 200 nm in film thickness was formed. Thepeak wavelength of the laser light emitted by the semiconductor laserelement 100 of Example 1 was about 530 nm.

EXAMPLE 2

A semiconductor laser element having the p-side semiconductor layer 4shown in FIG. 2B was prepared as Example 2. In other words, thesemiconductor laser element prepared was similar to Example 1 exceptthat an undoped Al_(0.05)Ga_(0.95)N layer of 100 nm in thickness (secondlayer 412B) was formed in addition to the first layer 412A as the p-sideintermediate layer 412, and the lower p-type semiconductor layer 431 wasformed to be 200 nm in thickness.

EXAMPLE 3

A semiconductor laser element having the p-side semiconductor layer 4shown in FIG. 2C was prepared as Example 3. In other words, thesemiconductor laser element prepared was similar to Example 2 exceptthat the second layer 412B was 200 nm in thickness and the lower p-typesemiconductor layer 431 was 100 nm in thickness.

EXAMPLE 4

A semiconductor laser element similar to Example 3 except that thethickness of the first layer 412A was 100 nm was prepared as Example 4.

EXAMPLE 5

A semiconductor laser element similar to Example 3 except that thethickness of the first layer 412A was 50 nm was prepared as Example 5.

EXAMPLE 6

A semiconductor laser element having the p-side semiconductor layer 4shown in FIG. 2D was prepared as Example 6. The semiconductor laserelement of Example 6 was similar to Example 3 except that the depth ofthe ridge 4 a was 270 nm. In other words, the semiconductor laserelements of Examples 1 to 5 were such that the lower end of the ridge 4a was positioned in the first layer 412A, whereas the semiconductorlaser element of Example 6 was such that the lower end of the ridge 4 awas positioned in the second layer 412B.

COMPARATIVE EXAMPLES 1-4

A semiconductor laser element similar to Example 1 except that the depthof the ridge 4 a was 270 nm was prepared as Comparative Example 1. Inother words, in the semiconductor laser element of Comparative Example1, the ridge 4 a was formed such that the lower end of the ridge ispositioned in the lower p-type semiconductor layer 431. Semiconductorlaser elements similar to Comparative Example 1 except that the p-sideintermediate layer 412 has different film thicknesses were prepared asComparative Examples 2 and 3. The film thickness of the p-sideintermediate layer 412 was 300 nm in Comparative Example 2, and 400 nmin Comparative Example 3. A semiconductor laser element similar toComparative Example 1 except for not having a p-side intermediate layer412 was prepared as Comparative Example 4. In other words, in all of thesemiconductor laser elements of Comparative Examples 1 to 4, the lowerend of the ridge 4 a was positioned in the lower p-type semiconductorlayer 431. The shortest distance from the active layer 3 to the electronbarrier layer 42 becomes larger in the order of Comparative Examples 4,1, 2, and 3. The peak wavelength of the laser light emitted by any ofComparative Examples 1 to 4 was about 525 nm.

Test Result 1

FIG. 8 shows the I-L characteristics of the semiconductor lasers ofComparative Examples 1 to 4. In the graph shown in FIG. 8, thehorizontal axis represents electric current and the vertical axisrepresents light output. In FIG. 8, the thin solid line representsComparative Example 1, the thin broken line represents ComparativeExample 2, the thick broken line represents Comparative Example 3, andthe thick solid line represents Comparative Example 4. As shown in FIG.8, for Comparative Examples 1 to 4, all of which had the lower end ofthe ridge 4 a positioned higher than the electron barrier layer 42, theI-L characteristics became more unstable as the shortest distance fromthe active layer 3 to the electron barrier layer 42 increased.Comparative Examples 1 and 2 had higher light output and higher slopeefficiency than Comparative Example 4, but had kinks occurring in theI-L characteristic curves. In the semiconductor laser elements ofComparative Examples 1 and 2, because the distance from the active layer3 to the lower end of the ridge 4 a was increased by disposing thep-side intermediate layer 412, the effective refractive index differencebetween inside and outside of the ridge 4 a was reduced as compared toComparative Example 4. This is believed to have made the horizontaltransverse mode of the semiconductor laser elements of ComparativeExamples 1 and 2 unstable to allow the kinks to occur. The semiconductorlaser element of Comparative Example 3 that had a thicker p-sideintermediate layer 412 not only allowed kinks to occur, but also had alower light output than the semiconductor laser element of ComparativeExample 4. As described above, when the lower end of the ridge 4 a waspositioned higher than the electron barrier layer 42, the I-Lcharacteristics were unstable, allowing kinks to occur, even with anattempt to improve the efficiency by disposing a p-side intermediatelayer 412.

Test Result 2

FIG. 9A shows the I-L characteristics and FIG. 9B shows the I-Vcharacteristics of the semiconductor laser elements of Examples 1 to 3.In the graph shown in FIG. 9A, the horizontal axis represents electriccurrent, and the vertical axis represents light output. In the graphshown in FIG. 9B, the horizontal axis represents electric current andthe vertical axis represents voltage. In both FIG. 9A and FIG. 9B, thethin solid line represents Example 1, the thick solid line representsExample 2, and the broken line represents Example 3. Examples 1-3 andComparative Examples 1-3 will be compared first by using FIG. 8 and FIG.9A. The shortest distance from the active layer 3 to the electronbarrier layer 42 in Example 1 is the same as that in Comparative Example1; that in Example 2 is the same as that in Comparative Example 2; thatin Example 3 is the same as that in Comparative Example 3. As can beunderstood from FIG. 8 and FIG. 9A, in the cases of Comparative Examples1 to 3, the I-L characteristics became more unstable as the distancefrom the active layer 3 to the electron barrier layer 42 increased. InExamples 1 to 3, on the other hand, the I-L characteristics weresubstantially the same regardless of the distance from the active layer3 to the electron barrier layer 42. This is believed to be attributableto enhanced optical confinement in the transverse direction by the moredeeply formed ridge 4 a in the semiconductor laser elements 100 ofExamples 1 to 3, which stabilized the horizontal transverse mode.Comparative Examples 1 to 3 had a higher light output than Examples 1 to3, but this is because Comparative Examples 1 to 3 had a shorteremission wavelength, i.e., it cannot be said that the differences in thedepth of the ridge 4 a caused the differences in the light output.

The film thickness of the lower p-type semiconductor layer 431 becomessmaller in the order of Example 1, Example 2, and Example 3. As shown inFIG. 9B, reducing the thickness of the lower p-type semiconductor layer431 was confirmed to reduce the voltage for driving the semiconductorlaser elements 100. This is believed to be because the lower p-typesemiconductor layer 431 being an AlGaN layer had a relatively highseries resistance. The second layer 412B is also an AlGaN layer, anddisposing such an undoped AlGaN layer in the p-side semiconductor layer4 could occasionally increase electrical resistance to thereby reducehole injection probability, but this effect was not observed in theresults shown in FIG. 9A and FIG. 9B. This is believed to beattributable to band bending in the second layer 412B during voltageapplication, which facilitated hole injection. In addition, it isbelieved that the first part 41 is filled with the electrons, whichoverflowed from the active layer 3 during voltage application.

Test Result 3

FIG. 10A shows the I-L characteristics and FIG. 10B shows the I-Vcharacteristics of the semiconductor laser elements of Examples 3 to 5.In the graph shown in FIG. 10A, the horizontal axis represents electriccurrent, and the vertical axis represents light output. In the graphshown in FIG. 10B, the horizontal axis represents electric current, andthe vertical axis represents voltage. In both FIG. 10A and FIG. 10B, thebroken line represents Example 3, the solid line represents Example 4,and the one-dot chain line represents Example 5. The thickness of thefirst layer 412A becomes smaller in the order of Examples 3, 4, and 5.As shown in FIG. 10A, reducing the film thickness of the first layer412A was confirmed to increase the slope efficiency. As described above,keeping the electron barrier layer 42 and the second part 43 thatcontain a p-type impurity at a distance from the active layer 3 canreduce light leakage Fp to the second part 43 having a large freecarrier absorption loss. On the other hand, electron overflow from theactive layer 3 increases as the electron barrier layer 42 becomes moredistant from the active layer 3. It is believed that reducing the filmthickness of the first layer 412A that has less impact on opticalconfinement than the p-side composition graded layer 411 and the secondlayer 412B can reduce the light leakage to the second part 43 whileattenuating an electron overflow increase, thereby increasing the slopeefficiency. As shown in FIG. 10B, moreover, it was confirmed that achange in the thickness of the first layer 412A hardly affected thedrive voltage.

Test Result 4

FIG. 11A shows the I-L characteristics and FIG. 11B shows the I-Vcharacteristics of the semiconductor laser elements of Examples 3 and 6.In the graph shown in FIG. 11A, the horizontal axis represents electriccurrent, and the vertical axis represents light output. In the graphshown in FIG. 11B, the horizontal axis represents electric current, andthe vertical axis represents voltage. In both FIG. 11A and FIG. 11B, thebroken line represents Example 3, and the solid line represents Example6. Although the depth of the ridge 4 a in Example 6 is smaller thanExample 3, they demonstrated similar characteristics, as shown in FIG.11A and FIG. 11B.

What is claimed is:
 1. A semiconductor laser element comprising: ann-side semiconductor layer formed of a nitride semiconductor; an activelayer disposed on or above the n-side semiconductor layer and formed ofa nitride semiconductor; and a p-side semiconductor layer disposed onthe active layer, formed of a nitride semiconductor, and comprising: anundoped first part disposed in contact with an upper face of the activelayer and comprising at least one semiconductor layer, an electronbarrier layer disposed in contact with an upper face of the first part,containing a p-type impurity, and having a band gap energy that islarger than a band gap energy of the first part, and a second partdisposed in contact with an upper face of the electron barrier layer andcomprising at least one p-type semiconductor layer containing a p-typeimpurity, wherein at least a portion of the p-side semiconductor layerforms a ridge projecting upward and having an upper face and a lowerend, wherein the first part comprises: an undoped p-side compositiongraded layer in which a band gap energy increases towards the electronbarrier layer, and an undoped p-side intermediate layer disposed on orabove the p-side composition graded layer, and wherein the lower end ofthe ridge is positioned at the p-side intermediate layer.
 2. Thesemiconductor laser element according to claim 1, wherein the secondpart comprises: an upper p-type semiconductor layer at which the upperface of the ridge is positioned, and a lower p-type semiconductor layerdisposed between the upper p-type semiconductor layer and the electronbarrier layer and having a band gap energy that is larger than a bandgap energy of the upper p-type semiconductor layer.
 3. The semiconductorlaser element according to claim 1, wherein the p-side intermediatelayer comprises: an undoped first layer having a band gap energy that islarger than an average band gap energy of the p-side composition gradedlayer, but smaller than the band gap energy of the electron barrierlayer, and an undoped second layer having a band gap energy that islarger than the band gap energy of the first layer, but smaller than theband gap energy of the electron barrier layer.
 4. The semiconductorlaser element according to claim 3, wherein the first layer and thesecond layer are single composition layers, and the lower end of theridge is positioned at the first layer or the second layer.
 5. Thesemiconductor laser element according to claim 3, wherein the firstlayer is a GaN layer.
 6. The semiconductor laser element according toclaim 3, wherein the second layer is an AlGaN layer.
 7. Thesemiconductor laser element according to claim 1, wherein the p-sidecomposition graded layer includes a plurality of sublayers that aredifferent in composition, wherein a lowermost sublayer among theplurality of sublayers of the p-side composition graded layer is made ofIn_(a)Ga_(1-a)N (0<a<1), and wherein an uppermost sublayer among theplurality of sublayers of the p-side composition graded layer is made ofIn_(z)Ga_(1-z)N (0≤z<a).
 8. The semiconductor laser element according toclaim 1, wherein a thickness of the first part is at least 400 nm. 9.The semiconductor laser element according to claim 1, wherein a shortestdistance from the lower end of the ridge to the electron barrier layeris larger than a shortest distance from the upper face of the ridge tothe electron barrier layer.
 10. The semiconductor laser elementaccording to claim 1 further comprising: a p-electrode disposed on theupper face of the ridge, wherein the p-electrode is a transparentconductive film having a refractive index that is smaller than arefractive index of the second part.
 11. The semiconductor laser elementaccording to claim 1, wherein the semiconductor laser element isconfigured to emit laser light having a wavelength of at least 530 nm.12. The semiconductor laser element according to claim 1, wherein thefirst part comprises an uppermost layer in contact with a lower face ofthe electron barrier layer, wherein the second part comprises alowermost layer in contact with the upper face of the electron barrierlayer, and wherein the lowermost layer of the second part has a band gapenergy that is smaller than a band gap energy of the uppermost layer.13. A semiconductor laser element comprising: an n-side semiconductorlayer formed of a nitride semiconductor; an active layer disposed on orabove the n-side semiconductor layer and formed of a nitridesemiconductor; and a p-side semiconductor layer disposed on the activelayer, formed of a nitride semiconductor, and comprising: an undopedfirst part disposed in contact with an upper face of the active layerand comprising at least one semiconductor layer, an electron barrierlayer disposed in contact with an upper face of the first part,containing a p-type impurity, and having a band gap energy that islarger than a band gap energy of the first part, and a second partdisposed in contact with the upper face of the electron barrier layerand comprising at least one p-type semiconductor layer containing ap-type impurity, wherein at least a portion of the p-side semiconductorlayer forms a ridge projecting upward and having an upper face and alower end, wherein the second part has a thickness that is smaller thana thickness of the first part, and wherein the lower end of the ridge ispositioned at the first part.
 14. The semiconductor laser elementaccording to claim 13, wherein the thickness of the first part is atleast 400 nm.
 15. The semiconductor laser element according to claim 13,wherein a shortest distance from the lower end of the ridge to theelectron barrier layer is larger than a shortest distance from the upperface of the ridge to the electron barrier layer.
 16. The semiconductorlaser element according to claim 13 further comprising: a p-electrodedisposed on the upper face of the ridge, wherein the p-electrode is atransparent conductive film having a refractive index that is smallerthan a refractive index of the second part.
 17. The semiconductor laserelement according to claim 13, wherein the semiconductor laser elementis configured to emit laser light having a wavelength of at least 530nm.
 18. The semiconductor laser element according to claim 13, whereinthe first part comprises an uppermost layer in contact with a lower faceof the electron barrier layer, wherein the second part comprises alowermost layer in contact with the upper face of the electron barrierlayer, and wherein the lowermost layer has a band gap energy that issmaller than a band gap energy of the uppermost layer.
 19. A method formanufacturing a semiconductor laser element, the method comprising:forming an n-side semiconductor layer on or above a substrate; formingan active layer on or above the n-side semiconductor layer; forming anundoped first part on an upper face of the active layer, the undopedfirst part comprising at least one semiconductor layer; forming anelectron barrier layer on an upper face of the first part, the electronbarrier layer being doped with a p-type impurity and having a band gapenergy that is larger than a band gap energy of the first part; forminga second part on an upper face of the electron barrier layer, the secondpart comprising at least one p-type semiconductor layer doped with ap-type impurity; and forming a ridge projecting upward by partiallyremoving a portion of the p-side semiconductor layer including a portionof the first part, a portion of the electron barrier layer, and aportion of the second part, wherein said forming the undoped first partcomprises: forming an undoped p-side composition graded layer in whichthe band gap energy increases away from the active layer, and forming anundoped p-side intermediate layer above the p-side composition gradedlayer, and wherein, in said forming the ridge, the p-side semiconductorlayer is partially removed such that a lower end of the ridge ispositioned at the p-side intermediate layer.
 20. A method formanufacturing a semiconductor laser element, the method comprising:forming an n-side semiconductor layer on or above a substrate; formingan active layer on or above the n-side semiconductor layer; forming anundoped first part on an upper face of the active layer, the undopedfirst part comprising at least one semiconductor layer; forming anelectron barrier layer on an upper face of the first part, the electronbarrier layer being doped with a p-type impurity and having a band gapenergy that is larger than a band gap energy of the first part; forminga second part on an upper face of the electron barrier layer, the secondpart comprising at least one p-type semiconductor layer doped with ap-type impurity; and forming a ridge projecting upward by partiallyremoving a portion of a p-side semiconductor layer including a portionof the first part, a portion of the electron barrier layer, and aportion of the second part, wherein, in said forming the second part,the second part is formed so as to have a thickness that is smaller thana thickness of the first part, and wherein, in said forming the ridge,the p-side semiconductor layer is partially removed such that a lowerend of the ridge is positioned at the first part.